If you're asking "why didn't the people before then work less?" the answer is "because productivity gains hadn't yet made that possible".
Just to expand on that a bit, in case it isn't fully clear:
There is a level of productivity per worker required to support everyone in the society. This level fluctuates some with the overall standard of living, but does not vary with total productivity, population, or GDP. Let's call this level P, for Parity. (And let's assume it's not "just barely enough to support everyone", but "enough to support them comfortably and reliably, with a decent buffer".)
Once the level of productivity per worker passes certain thresholds—multiples of P—the total amount of work required to maintain the society at the same level drops. More work produces surplus. That surplus can be then used in a variety of ways. One of those ways is by reducing the amount of work being done. So, for instance, if the productivity level reaches 2P, then every worker can work half the amount of time they were working before, and still be producing enough to fully provide for everyone. If it reaches 3P, then every worker can work 1/3 the amount of time, and so on.
If, instead, the surplus is captured by the wealthy, the workers don't see benefit, and inequality grows.
I don't know exactly when real-world productivity got enough higher than P that we could realistically start reducing worked hours, but it definitely did so at some point in the last several decades. I'd say that at this point, just as a rough estimate, we're probably somewhere between 1.5P and 2P, but that's not really my field of expertise. But because the wealthy have captured approximately all productivity gains above the level we were at in 1980, they have seen their wealth massively increase, while the rest of us have just been scraping by.